Quite a few already have stories, being already-existing gods or goddesses. Look at Tiamat. Tiamat was the leader of the older generation of Mesopotamian gods who tried to overthrow the younger generation, lead by Lord Marduk. Marduk wound up killing Tiamat and cutting her up, using her pieces to create the earth. That's Tiamat's story.
Then we have Zeus. Enough said. I'm sure we all know who Zeus is. Same with Charon.
Cybele is another goddess of the Romans. She was the Roman counterpart to the Greek Rhea, just as Jupiter is the Roman counterpart to the Greek Zeus. Cybele was the goddess of fertility.
These are but a few of the examples.
...Wow, sorry for the ancient mythology lesson.